Immunoglobulin E (IgE) associates with allergic responses, but we found that its serum levels are enhanced in patients with acute myocardial infarction or unstable angina pectoris; they are nearly double those in patients with stable angina pectoris or without coronary heart disease. Both IgE and its high-affinity receptor FcR1 were expressed highly in human atherosclerotic lesions, localized to areas rich in macrophages (M), and colocalized to lesion smooth-muscle cells (SMCs) and endothelial cells (ECs). Preliminary experiments showed that in the absence of FcR1 -subunit, atherosclerosis-prone apolipoprotein E-deficient (Apoe-/-) mice developed 75% fewer lipid depositions in the thoracic-abdominal aorta and had a >50% reduction in atherosclerotic lesion sizes in the aortic arch, compared with Apoe-/- control mice. In vitro cell culture studies demonstrated that IgE induced activation of M, SMC, and EC mitogen-activated protein kinase (MAPK) pathway, along with enhanced inflammatory molecule production and apoptosis. Surprisingly, IgE stimulated a complex formation between FcR1 -chain and Toll-like receptor 4 (TLR4). Both FcR1 -subunit and TLR4 are necessary for IgE-induced M cytokine and chemokine expression and apoptosis. Furthermore, these activities of IgE on M associate with activation of Na+-H+ exchanger NHE1, followed by extracellular pH reduction. These novel preliminary findings are consistent with prior observations that in human atherosclerotic lesions, apoptotic cells cluster around lipid-rich necrotic cores, where the pH values are significantly lower (pH 7.150.01) than in other areas of the same lesions (pH 7.550.32, P=0.0001). Therefore, we hypothesize that IgE contributes to atherosclerosis by regulating M, vascular SMC, and EC inflammation and apoptosis, and that these processes depend on FcR1 and TLR4 interaction as well as on NHE1 activation. We propose two specific aims to test this hypothesis: 1). To examine the molecular mechanisms by which IgE stimulates M and vascular SMC and EC inflammation and apoptosis. 2). To examine whether inhibition or deficiency of IgE or IgE receptor FcR1 reduces atherosclerosis, and to test the importance of IgE-mediated M activation in atherogenesis. We anticipate that the experiments proposed in both aims will establish an important and novel role of IgE in M and vascular cell pathobiology, and a direct participation of IgE in atherogenesis.